A highly sensitive, rapid and facile method of fluorophotometry is described for the determination of phosphate using the ion pair formation of molybdophosphate with Rhodamine 6G. This ion pair is collected on a membrane filter made of cellulose nitrate when the sample solution is adjusted to 0.35 mol dm-3 in sulfuric acid, 1.9 mmol dm-3 in ammonium molybdate and 3.1-mu-mol dm-3 in Rhodamine 6G, and it is filtered through the membrane filter under suction. The membrane filter together with the ion pair is dissolved into Methyl Cellosolve. The fluorescence intensity of this solution is measured at 555 nm with the excitation at 535 nm. The calibration curve of phosphate is linear from 5 to 100 ng of phosphorus, with the correlation coefficient of 0.999; the relative standard deviation was 1.6% for 100 ng of phosphorus (n = 12). Although arsenic(V) causes positive error, no ions, including silicate, commonly existing in natural water interfere with the determination of phosphate. The proposed method was applied to the determination of phosphate-phosphorus in water samples and satisfactory results were obtained.
